1. Field of the Invention
The present invention relates to a radio communications system and, more particularly, to a resource allocation technique for a common channel in a radio communications system.
2. Description of the Related Art
In wideband-code division multiple access (W-CDMA), which is a third-generation mobile communications system, a mobile station (UE (user equipment)) in CELL_FACH state does not have a specified base station but selects an optimal base station every time it performs communication. Moreover, since a mobile station in CELL_FACH state is not assigned a dedicated channel, it performs uplink data transmission/downlink data reception by using a common channel. The operations of a random access channel (RACH), which is an uplink common channel, are defined by 3GPP (Third Generation Partnership Project, which is a project for standardization of 3G mobile communications systems) specifications (see 3GPP TS25.214 v7.5.0, 3GPP TS25.321 v7.2.0, 3GPP TS25.331 v7.3.0, and 3GPP TS25.211 v7.2.0). Hereinafter, the operations of the uplink common channel, RACH, will be described briefly with reference to FIGS. 1 to 3.
FIG. 1 is a block diagram showing a structure of a mobile communications system in general. Here, to avoid complicating the description, it is assumed that a plurality of mobile stations (here, 20.1 to 20.4) are located within the cell of a base station 10, with each mobile station being in CELL_FACH state, and the base station 10 being connected to an upper network device 30. Note that, in the following description, an expression “mobile station 20” will be used when an arbitrary mobile station is indicated.
FIG. 2 is a diagram of the structure of the uplink common channel, RACH. FIG. 3A is a RACH sequence diagram, and FIG. 3B is a table showing an example of preamble-part code data components and base station's responses. Referring to FIG. 2, an uplink communication has a RACH message part for transmitting a message body and a preamble part for providing timing before the transmission of the RACH message part. For a downlink communication, an acquisition indicator channel (AICH) is provided, which is a downlink channel for responding to a preamble part received from a mobile station.
In RACH, spreading codes called “preamble signature Csig,s” and “preamble scrambling code Sr-pre,n” as described below are used. The preamble scrambling code Sr-pre,n is a code for cell identification, which is notified by a base station. The preamble signature Csig,s is one randomly selected from among predetermined preamble signatures Csig,1, Csig,2, . . . Csig,n by each mobile station and has a one-to-one association with a channelization code, which will be described later.
The code data Cpre,n,s of the RACH preamble part is composed of a preamble signature Csig,s and a preamble scrambling code Sr-pre,n, as represented by the following equation 1 (see FIG. 3B).
                                          C                          pre              ,              n              ,              s                                ⁡                      (            k            )                          =                                            S                                                r                  ⁢                                      -                                    ⁢                  pre                                ,                n                                      ⁡                          (              k              )                                ×                                    C                              sig                ,                s                                      ⁡                          (              k              )                                ×                      e                          j              ⁡                              (                                                      π                    4                                    +                                                            π                      2                                        ⁢                    k                                                  )                                                                        (        1        )            where k=0, 1, 2, 3, . . . , and 4095, Cpre,n,s is preamble-part code data, Sr-pre,n is a preamble scrambling code, and Csig,s is a preamble signature.
On AICH, a response (ACK/NACK) to a preamble is transmitted to the mobile station, using a code pattern corresponding to the preamble signature of the preamble.
The RACH message part is composed of a RACH message control part for transmitting a control signal and a RACH message data part for transmitting data. The RACH message part is coded by using a channelization code associated with the preamble signature, then I/Q-multiplexed, and then further coded by using a scrambling code associated with the preamble scrambling code.
As shown in FIGS. 2 and 3A, a mobile station 20 first generates preamble-part code data by using a preamble scrambling code notified from the base station 10 and a preamble signature the mobile station 20 has randomly selected itself. The mobile station 20 transmits the preamble-part code data to the base station 10 with transmission power of an initial value, which is calculated from the amount of the received power of a pilot channel from the base station 10.
In response to the received preamble, the base station 10 transmits a response to the mobile station 20 by using AICH. In this transmission, the base station 10 also transmits to this mobile station 20 information about the responses to all preamble signatures. For example, the base station 10 notifies the mobile station 20 of information including the preamble signatures Csig,1, Csig,2, . . . , Csig,m and responses (ACK, NACK, or No ACK) to these preamble signatures shown in FIG. 3B.
For example, to a preamble signature that is used in a preamble successfully received by the base station 10, a response “ACK” indicates that a mobile station which has selected this preamble signature is allowed to transmit a RACH message at a predetermined timing. When a mobile station is not allowed to, “NACK” is notified. Moreover, to a preamble signature that is not used in the successfully received preamble, “No ACK” is set as a response on AICH. Note that the base station 10 sometimes does not transmit a response over AICH when there is no preamble successfully received.
Upon receipt of a response over AICH, when ACK is the response to the preamble signature used in the preamble transmission, then the mobile station 20 determines a RACH message part transmission profile by using some method, which will be described later, and transmits data to the base station 10. If NACK is the response to the preamble signature used in the preamble transmission, the mobile station 20 starts a preamble transmission procedure again in a predetermined length of time. When No ACK is the response to the preamble signature used in the preamble transmission, the mobile station 20 determines that the base station 10 has failed to receive the preamble last transmitted and, if the number of retransmissions does not reach an upper limit yet, retransmits the preamble with transmission power increased by a predetermined amount.
Incidentally, as shown in FIG. 2, a minimum retransmission interval τp-p,min between retransmissions of a preamble part, an interval τp-a between a transmission of a preamble part and a transmission of a response over AICH, and an interval τp-m between a transmission of a preamble part and a transmission of a RACH message part are individually predetermined.
The RACH message part transmission profile includes an offset value of the transmission power of the RACH message part (RACH message part transmission power offset value), a scrambling code, a channelization code, and a transmission timing. The transmission power offset value of a RACH message part can be obtained from the value of the transmission power of the preamble last transmitted by the mobile station 20 before the receipt of ACK over AICH. Moreover, the scrambling code has a one-to-one correspondence with the preamble scrambling code used in the preamble transmission, and the channelization code has a one-to-one correspondence with the preamble signature used in the preamble transmission. Furthermore, the transmission timing is determined based on the time when the preamble part was transmitted, because the interval τp-m between a preamble part transmission and a RACH message part transmission is predetermined as shown in FIG. 2.
The RACH message part is composed of the RACH message control part and RACH message data part as mentioned above already. The transmission power values of these parts can be calculated by using the following equations (2) and (3), respectively.Transmission power of RACH message control part=Ppreamble,tx×ΔPp-m  (2)Transmission power of RACH message data part=Ppreamble,tx×TF_offset  (3)where Ppreamble,tx is the value of the transmission power of the preamble last transmitted by the mobile station 20 before the receipt of ACK over AICH, ΔPp-m is a transmission power offset value to Ppreamble,tx, and TF_offset is a transmission power offset value corresponding to a data format (TF: Transport Format) in use.
The base station 10 notifies the mobile station 20 of a set of TFs (TFS: Transport Format Set) that are available to mobile stations in the cell for transmission of a RACH message data part, transmission power offset values TF_offset corresponding to the individual TFs, and a transmission power offset value ΔPp-m to the value of the transmission power of the preamble (Ppreamble,tx) r as default profile information, which is updated at predetermined time intervals.
The mobile station 20 compares the amount of uplink transmission data buffered in the mobile station 20, Buffer_size, with the above-mentioned available TFS and selects the smaller data size. When the sum of the RACH message control part transmission power value and the RACH message data part transmission power value, calculated using the equations (2) and (3), exceeds a preset maximum value MAX_Tx of the transmission power of the RACH message part, the mobile station 20 reselects such a TF that the sum of the RACH message control part transmission power value and the RACH message data part transmission power value does not exceed the maximum transmission power value MAX_Tx. The mobile station 20 then transmits data by using the selected TF.
The uplink common channel, RACH, is defined as described above in 3GPP T525.214 v7.5.0, 3GPP T525.321 v7.2.0, 3GPP TS25.331 v7.3.0, and 3GPP TS25.211 v7.2.0. Further, in Text Proposal R2-071076 (3GPP TDoc (written contribution) at meeting), the enhanced peak rate, enhanced line throughput, and a function (Enhanced CELL_FACH) for reduced delay of a downlink communication in CELL_FACH state are defined.
To achieve the enhanced peak rates, enhanced line throughputs, and reduced delays of both uplink and downlink communications in CELL_FACH state defined in 3GPP TDoc R2-071076, it is also necessary to enhance the peak rate and line throughput of RACH and to reduce the delay thereof.
If every mobile station in a cell performs a preamble transmission by using common resource information and receives ACK from a base station over AICH in response to the transmission as described with reference to FIG. 2, then RACH message part transmissions are performed after a predetermined period (τp-m) has passed since the respective preamble transmissions. It can be thought that the occurrence of transmission data in each mobile station is a random event. Accordingly, if each mobile station performs a preamble transmission at the timing of the occurrence of transmission data, there are some occasions when preamble transmissions by the mobile stations concentrate. If preamble transmissions concentrate as described above, data transmissions also concentrate after a lapse of the period τp-m, resulting in the uplink falling in overloaded state. This may cause problems such as degradation in link quality and frequent failures of data transmission. Although these problems are minor when the transmission rate is low, the problems are significant particularly when the transmission rate is high.